Part 1: Amino acids and amides: two groups, one molecule

Predict acid, alkali and esterification reactions by examining each functional group, then recognise amide connectivity.

Locate both functional groups before predicting a reaction

An α-amino acid has the general structure H₂N–CH(R)–COOH: the NH₂ group is on the carbon next to the carboxyl carbon. The side chain R determines which amino acid it is. Glycine has R = H and alanine has R = CH₃.

The COOH group can donate H⁺ and the NH₂ group can accept H⁺, so an amino acid can react with both acids and bases. This is amphoteric behaviour. A question may include additional acidic or basic side-chain groups; count them rather than assuming every amino acid has exactly one of each.

Useful explanatory extension: in many aqueous and solid-state conditions amino acids exist predominantly as zwitterions, with NH₃⁺ and COO⁻ on the same molecule. A net zero charge does not mean the molecule contains no charged groups. Detailed isoelectric-point calculations are not a required recall item here.

Let the reagent select the reacting group

With acid, the amine group is protonated to NH₃⁺. Under sufficiently acidic conditions the carboxyl group is COOH, so the simple amino acid carries net +1 charge. With alkali, COOH becomes COO⁻ and the amine is unprotonated NH₂ under sufficiently alkaline conditions, giving net −1 for the simple one-amine/one-acid example.

When asked to draw a salt, include the counterion as appropriate. The nitrogen in NH₃⁺ has four bonds, including its bond to carbon; neutral NH₃ attached to carbon would be a valency error. Similarly, draw both oxygens in COO⁻.

If starting from a zwitterion, acid protonates COO⁻ and alkali removes a proton from NH₃⁺. The final structures are the same limiting acidic/alkaline forms. The chosen starting representation changes the way you write the intermediate proton transfer, not the overall chemistry.

Modify COOH while tracking the amine’s protonation

The carboxylic acid group can form an ester with an alcohol under acid-catalysed conditions. Using ethanol changes –COOH to –COOCH₂CH₃. The amino-acid carbon skeleton and side chain remain.

In the acidic reaction mixture the amino group is protonated. If a question asks for the neutral organic structure after appropriate work-up, show NH₂; if it asks for the species in acid, show NH₃⁺. State your assumed work-up when the distinction matters.

For alanine, the neutral ester connectivity is H₂NCH(CH₃)COOCH₂CH₃. It contains five carbons: three from alanine and two from ethanol. Attaching ethyl directly to nitrogen would be an amine-alkylation product, not esterification.

Recognise the C(=O)–N link

A primary amide is RCONH₂; a secondary amide is RCONHR′. In each, N is directly bonded to the carbonyl carbon. CH₃CONH₂ is ethanamide, whereas CH₃CH₂NH₂ is ethanamine. The carbonyl changes the functional-group class.

A peptide link is an amide link, –C(=O)–NH–, formed between the carboxyl group of one amino acid and the amine group of another. It is not an ester link –C(=O)–O– and does not contain an extra oxygen between carbonyl carbon and N.

For a dipeptide from glycine and alanine, H₂NCH₂CONHCH(CH₃)COOH and H₂NCH(CH₃)CONHCH₂COOH are different sequences. Joining the two monomers consumes OH from one COOH and H from one NH₂, giving one H₂O for one link. Free terminal groups remain.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources